1. Field of the Invention
This invention relates to the attachment of printed circuit board-mounted, heat generating electrical components to heat sinks. In particular, to the attachment of such electrical components which have heat transfer tabs to heat sink housings using spring type members.
2.Description of Related Art
One method by which tabbed printed circuit board-mounted, heat generating electrical components have been attached to heat sinks has involved individually attaching each device to the heat sink with a screw through the hole in the tab of the electronic device, and utilizing an electrically insulating thermally conductive film between the heat sink and the device tab. A nut is tightened onto the screw to provide the compressive force required to provide adequate compression of the film to maximize heat transfer from the device to the heat sink through the film. There are many variations of this technique, such as using a threaded hole in the heat sink rather than a nut. Problems arise with this general manner of attachment because the tension created by the screw may be relieved by material creep in the insulating washer or grommet, or the screw may back out under certain operating conditions, such as where the device is subject to vibration or temperature cycling. Another disadvantage is that the large number of parts that must be manually assembled when there are a number of heat generating components in an assembly, and it is difficult to automate the production of such attachments for high volume manufacturing.
In addition to the various uses of attachment screws, numerous attachment techniques have been developed which use spring members. A number of these attachments use clips with multiple spring fingers, as can be seen by reference to U.S. Pat. Nos. 4,922,601; 4,891,735; 4,872,089; 4,845,590; 4,707,726; 4,674,005; 4,605,986; and 4,479,140, for example. Difficulties are encountered with these techniques for various reasons. To achieve a compact assembly, the springs must achieve high forces at low deflections, and this means that the springs must be manufactured specifically for an individual device and changing the device may cause the spring to exert too much or too little force since stiff springs have force constants which create a very narrow range of deflections within which the proper pressure will be applied. Furthermore, there is often a need for special tools to insure that the device or the spring is not damaged during assembly. Historically, electrically insulating and thermally conductive pads require 300-500 psi for effective heat transfer; but, this requires a material to be disposed between the spring and the device to distribute the spring force on the device to prevent the spring from damaging the device. Some arrangements require that the heat sink be a separate part from the housing, and others require numerous unique parts that need to be manually assembled.
Canted coil springs have the attribute that, unlike conventional springs which apply a force which varies as a function of deflection, they apply an essentially constant force over a large deflection range. This type of spring is well known for use in electronic devices. Most commonly, they are utilized as a means to hold a printed circuit board in place in proper alignment. In this regard, reference can be made to U.S. Pat. Nos. 5,092,781 and 5,069,627, as well as to page 3.1B-2 of Bat Seal Engineering Company catalog no. LE-44D.
Additionally, canted springs are known for use as electrical and thermal conductors. For example, in U.S. Pat. No. 4,993,482, a thermally conductive, canted coil spring is used to provide a low force, thermal path between heat generating electronic components and a heat sink in close proximity. In this arrangement, the spring is flattened to increase the contact between the canted spring and the components and heat sink and thereby decrease the thermal resistance.
Thus, there is a need for a heat sink attachment which will enable the use of low force springs in order to increase the range of permissible spring deflections, thereby increasing the tolerances and the variety of heat sinks which may be attached, and to eliminate the need for intervening materials for distributing the spring force. Additionally, a need also exists for an arrangement which will not require numerous parts that require manual assembly.